The shell has a procedure for searching for executable when you type them in. If you type in a command with slashes, like /bin/cp then it tries to run the named program: cp out of the /bin directory. If you just type cp on its own, then it tries to find the cp command in each of the sub-directories of your PATH. To see what your PATH is, just type
You will see a colon separated list of directories. Note that the current directory . is not listed. It is very important that the current directory not be listed for security reasons. To execute a command in the current directory, we hence always type ./
To append for example a new directory /opt/gnome/bin to your PATH, do
you can also do this in one line: export PATH="$PATH:/opt/gnome/bin"